“SUDDEN COMMENCEMENTS” IN THE GREENWICH MAGNETIC RECORDS (1879-1944) AND RELATED SUNSPOT DATA

Abstract

Summary Part I: Statistics of “sudden commencements” -their average amplitude, direction of impulse, hourly and monthly frequencies-are derived from 681 cases identified on the Greenwich magnetograms over six sunspot cycles. Besides SCs that mark the onset of many geomagnetic storms, there are frequent cases in which no appreciable disturbance follows. About half of all SCs have a small preliminary impulse (lasting about half a minute) in opposition to the main impulse, most commonly of three minutes' duration. The presence of this preliminary impulse seems subject to some diurnal effect. Both diurnal and seasonal effects appear in SC frequency occurrence. An unexpected result is a marked minimum around 8h-9h. SCs that do occur then are usually inverted (the normal main stroke is +ΔH, +ΔV and - ΔD). Details are tabulated for every SC recorded at Abinger from 1938 November to 1944 December for comparison elsewhere. Part II: SC occurrence inrelation to the sunspot cycle and to individual sunspots is investigated. Each of the four groups into which the data were divided gives a frequency curve similar to the mean sunspot curve. In this respect, small geomagnetic storms with no SC onset have a different distribution within the sunspot cycle. A marked tendency for SCs followed by storms to occur when big spots are about one day past the central meridian accords with a well-known relationship between the larger magnetic storms and the bigger sunspots. The association of SCs not followed by disturbance with respect to big sunspots appears largely fortuitous, but some effect from spots nearing the western limb should not be ruled out. An addendum gives the results of a sample comparison of SC pulses and other minor movements on the Abinger and Lerwick magnetograms. SCs as recorded at the two stations are very similar, but ΔV at Lerwick is normally negative. During inactive periods a remarkable similarity of very small detail is noted between these two stations which are over 600 miles apart.